I am new to Joomla, and have a template using Joomla 2.5. I have tried using my own PC to customise this template, and place onto my website, provided through a third party. However there seems to have been an issue with the MySQL install, and each time I uninstall MySQL, or try to reinstall Joomla, it still uses the old (and incorrect) root directory and password. Does anyone know how to resovle this?

Hello,
If you are not similar to Joomla or MySQL, PHP , Apache I will suggest to use Wamp server or Xampp server, and then install Joomla
It will easier for you to manage and work
If you faced any problem let us know

Every install for Joomla weather its 3.x or 2.5x I have cPanel up with MY SQLDatabases. Your hosting company should provide you with this. I use my own co-located server and I set this up for all my hosting accounts. In MYSQL you set up your username and database name. YOU MUST set the parameters for that account my clicking it. You set to ALL, then you continue install with the information you put in there.
IF your hosting company does NOT provide you with this contact me and I can help you set up a PERFECT JOOMLA hosting account with the newest Apache and the newest version of PHP. Your site will fly.... Meanwhile I can help you in any way I can.